2H, or deuterium, is a stable isotope of hydrogen. It contains one proton and one neutron in its nucleus, making it about twice as heavy as the most common hydrogen isotope, protium, which has no neutrons. Deuterium is naturally occurring and can be found in small amounts in water, where it forms heavy water (D2O). Deuterium is used in various scientific applications, including nuclear fusion research and as a tracer in chemical reactions. Its unique properties make it valuable in NMR spectroscopy and mass spectrometry, helping scientists study molecular structures and dynamics.